1. Field of the Invention
This invention relates to a heavy duty pneumatic radial tire for use in a construction vehicle or the like.
2. Description of Related Art
In general, the pneumatic radial tire mounted onto a construction vehicle or the like is frequently run on a bad road, rough road and the like scattering rocks and ground stone pieces thereon under a heavy load, so that cracking is caused at a widthwise end of a belt layer and is apt to grow into separation failure.
In order to suppress such a separation failure at the belt end, there has hitherto been proposed a radial tire comprising an auxiliary belt layer disposed between a belt layer and a carcass ply at a width narrower than that of the belt layer and an inclination cord angle smaller than that of the belt layer with respect to a tread center as disclosed in JP-B-3-23361. In this tire, a greater part of tension in a circumferential direction at an inflation of an internal pressure and under loading is born by the auxiliary belt layer, whereby the diameter growth of the tire is controlled while increasing the inclination cord angle in the belt layer to suppress the separation failure at the belt end.
However, when the auxiliary belt layer is disposed between the belt layer and the carcass ply, the rigidity at the central portion of the tread becomes fairly high and the rigidity at a shoulder portion relatively lowers, and hence the bending deformation of the shoulder portion is increased in the running under loading. Thus, when the radius of curvature of the outer profile of the tread in the tire is constant over a full region of the tread, the ground contact form under loading becomes ununiform or the ground contact length in the central portion of the tread is restricted, while the ground contact length in the shoulder portion increases. As a result, the wear resistance of the tire lower but also the heat generation quantity in both the shoulder portions increases, so that there is a problem that it is apt to cause separation failure at a belt end through heat.
It is, therefore, an object of the invention to provide a heavy duty pneumatic radial tire capable of easily improving the wear resistance and the resistance to separation at belt end.
According to the invention, there is the provision of in a heavy duty pneumatic radial tire comprising a carcass ply of substantially a toroidal shape containing a plurality of inextensible cords arranged side by side in a radial direction of the tire, a belt disposed at an outside of the carcass ply in the radial direction and comprised of two or more belt layers each containing a plurality of inextensible cords arranged at a given inclination cord angle with respect to a center S of a tread, at least one auxiliary belt layer disposed in a widthwise central portion of the belt at a width narrower than that of the belt and containing inextensible cords arranged at an inclination cord angle with respect to the tread center S smaller than that of the cord in the belt, and a tread arranged at an outside of the belt in the radial direction, an improvement wherein when the tire is mounted onto a design rim defined in TRA standard and inflated under an internal pressure corresponding to 10% of an air pressure defined in the TRA standard, a radius of curvature of an outer profile at both side regions of the tread (Rs) is made larger than a radius of curvature of the outer profile at a central region of the tread (Rc).
In the invention, the radius of curvature of the outer profile (Rs) at both side regions of the tread is made larger than that (Rc) at the central region of the tread, so that the outer profile at both the side regions approaches to a straight line and hence the bending deformation amount at the shoulder portion under loading is restricted to control the increase of the ground contact length to thereby provide substantially the uniform ground contact shape. As a result, the wear resistance of the tire is improved and also the heat generation quantity at both the shoulder portions is controlled to improve the resistance to separation at the belt end. In order to effectively control the growth of the tire diameter, it is favorable that the width and inclination cord angle of the auxiliary belt layer are 0.2xcx9c0.6 times a width of the tread and more than 0xc2x0 but not more than 15xc2x0, respectively.
And also, the above effects can be ensured when a boundary point K between each side region and the central region of the tread is located between an inner point P1 separated from the tread center S by 0.225 times a tread width T and an outer point P2 separated from the tread center S by 0.400 times the tread width T.
Furthermore, when the radius of curvature Rc of the outer profile in the central region is within a range of 1.8xcx9c5.5 times a section height Sh of the tire, it can be prevented that the bending deformation in the tread central portion (ground contact pressure) becomes excessively large or small, whereby the amount of bending deformation in the shoulder portion under loading can sufficiently be restricted.
Moreover, when the radius of curvature Rs of the outer profile in both the side regions is not less than 1.3 times the radius of curvature Rc of the outer profile in the central region, there can be prevented the increase of the wearing in the tread central portion.
In addition, when the auxiliary belt layer is arranged between the carcass ply and the belt, the resistance to separation at belt end can further be improved.